The JSON flat file sounds like variable length records where length is determined by parsing each record with global reader/writer locking?
It's like public storage where you have to sift through everyone else's crap to get to yours, every item is stored in a bulk cargo box and only one customer gets to store their stuff at a time.
If you're interested in db internals, here's a few algorithms that MySQL uses. Note this doesn't cover InnoDB which performs far better under high concurrency loads and offers row level locking, clustered indexes, an excellent caching algorithm, foreign key constraints with cascade, etc..etc...
The way I've stored things in flat files is to make use of the file system's lookup capabilities. I wouldn't suggest storing data that may be requested or written to by more than one client at once with a method such as this, either. I wouldn't try to use it like a C array or anything. One file per user or more makes sense. I've also used stuff like this for system admin scripts.
#get the file, decode, etc.
It's a quick and dirty caching method that has good persistence, of course, is relatively performant under low loads and easy to understand.
Thanks, I'll check out the MySQL thing, but I'm not actually intending to build my own database.